Why long haul microwave is in for the long haul
Could it be that long haul microwave is past its sell-by date? Ever since synchronous digital hierarchy (SDH) and synchronous optical networking (SONET) began to be replaced by packet traffic, people have questioned the need for long haul microwave in today’s networks. One CTO even put the question to me at our annual 5G Transport gathering. Having spent many years working with microwave and telecom networks, I can honestly say that long haul microwave is more relevant than ever in the packet networks of today.
Meeting user expectations
Today’s packet network users want to be connected at all times. They expect an ultra-high-speed connection for live video streaming, for example, but also like to enjoy low-bandwidth services such as Facebook, text messaging and ordinary voice calling. The trouble is that while optical fiber remains service providers’ first choice when it comes to mobile transport networks, in many places, there is no reasonably priced optical fiber available.
The next-best option has to be using the E-band to cover the distance to the optical grid, and if it’s too far away, then perhaps two to four times 56MHz or a 112MHz channel in the 23GHz spectrum band (or higher) will do the trick. But what to do when the distance is too great for these technologies to cover? When you have to span 10-120km, for example, and still need your 2-10Gbps backhaul capacity? This is where the modern microwave long haul systems working in the 4-13GHz frequency range come into play. With multiple channels and even multiple frequency bands in combination with adaptive modulation and layer 1 (L1) radio-link bonding, you can reach the capacity target and meet service-availability needs with long haul microwave.
The TDM harness
This might sound like a contradiction, but the removal of the TDM harness from the microwave link and the use of modern packet technology enable high network availability and capacity in a way that was never possible before. The traditional SDH-based long haul systems offered high availability at the price of lower capacity per channel compared with a modern packet link. In order to ensure high availability at all times, the link had to be designed for the worst-case scenario, and never offered more capacity than that. The problem of selective fading in these frequency bands was countered with a combination of space diversity and N+1 protection, with up to seven different channels sharing a single protection channel. If more than one channel was subject to heavy fading or equipment failure, the link was impacted anyway. With these solutions, you did all planning per channel since each channel always had to deal with the worst-case scenario on its own.
More capacity and availability with today’s packet network
In the packet network, the situation is very different: traffic is not handled per radio channel; it’s handled per packet! Now this creates many new possibilities for a long haul system. Each packet has its own QoS marking and is therefore handled independently of all other packets. The radio links cooperate within L1 radio-link groups with a combined and dynamic capacity when using adaptive hitless modulation. This means each channel offers the best and highest possible error-free capacity at all times. All packets are transported as fast as possible over a combined channel, and the order of transmission depends on the QoS priority settings. So high-priority traffic will always get through – even when capacity is severely limited. In the TDM network, the situation was very different: depending on which radio frequency channel was being used for the traffic, if the channel was impacted, all the traffic on it was lost.
Another positive aspect of the packet network is that every packet benefits from a kind of frequency diversity protection as well as the space diversity that is still present in most cases. The fact that the total capacity of the link is greater than the capacity needs of the high-priority traffic means that it adds even more protection to this part of the traffic being carried.
And so, if a traditional 7+1 SDH link was planned with a 99.995 percent availability target per link at 128 QAM modulation, an 8+0 packet link could simultaneously offer the seemingly impossible improvement of twice the capacity at 99.95 percent availability and an increase to 99.9999 percent availability for the really high-priority traffic. The latter is possible since the link can now be allowed to go all the way down to 4 QAM before it totally fails, and this still means that some 350Mbps of traffic can be upheld. In the traditional SDH solution, all links would have gone down long before this level was reached.
Why not enjoy the benefits?
So, what’s stopping us from making the most of all these benefits in a modern long haul system? To start with, we are in the habit of sticking with what we are used to doing: in other words, we prescribe one and only one level of availability of 99.999 percent for the microwave hop. In order to enjoy the benefits to the full, we need to change the way we think and prescribe two levels of availability: 99.9 percent for the full capacity of the link at 4096 QAM, for example, and 99.999 percent for 50 percent of the maximum capacity. This way, our long haul system will deliver more capacity during nearly the whole year, and for just nine hours, we will have less than full capacity – somewhere between full capacity and what the old SDH link offered. So, connectivity and full capacity can be offered more cost-effectively than fiber most of the time.
Long haul more essential than ever today
In a modern packet network, long haul microwave does indeed have an important role to play. It delivers more capacity, more availability and better protection today than in the old TDM networks. Long haul systems are even more essential today for rural and remote backhaul, as well as for closing fiber rings where fiber is too costly.
So if you are considering using microwave long haul for your suburban or rural area transport planning, I recommend you to check out this page for more information.